The invention is based on a coil having a core and windings which are arranged as conductor tracks on base layers.
Coils or transformers which are operated, in particular, at frequencies of more than 100 kHz and are designed as power components are considerably influenced, in terms of their electrical characteristics, by the skin effect. Since, depending on the frequency, the skin effect means that currents flow only in the outer layer of a conductor, wires or conductor tracks cannot be made appropriately thicker to deal with higher power levels so that, for example, copper braids have to be used instead of a wire. However, even when copper braids are used in coils of a convention types which are used in switched-mode power supplies at frequencies of, for example, 500 kHz, their temperatures are more than 100.degree. C. and their losses are several watts during operation. The use of copper sheet with an insulating layer located in between is more costly, and is likewise still subject to an extreme temperature rise of 60.degree. C. at 500 kHz.
Application Note Philips Magnetic Products, "Design of Planar Power Transformers" discloses the use of a board technique in a power transformer for relatively high frequencies. In this case, a plurality of double-sided boards are located one above the other, and each board contains a plurality of windings, whose inner end is plated through in order to connect the windings to one another. The boards have a hole in the centre, through which the core is passed, so that there is a closed magnetic circuit in the core.
The object of the present invention is to specify a coil of the type mentioned initially which has low electrical losses with compact dimensions.